Eight soldiers were undergoing training near the peak in Japan when the eruption caught them by surprise.

Twelve people, including eight soldiers, skiing on the slopes of a volcano near a famous hot spring resort in central Japan were injured by flying rocks during a sudden eruption, officials said, with one soldier dying from his injuries.

Twelve people, including eight soldiers, skiing on the slopes of a volcano near a famous hot spring resort in central Japan were injured by flying rocks during a sudden eruption, officials said, with one soldier dying from his injuries.

The eruption quickly darkened the ski slope at Mount Kusatsu-Shirane with black ash, as volcanic rocks rained down on gondolas and people skiing down the slopes, sending them desperately seeking shelter at a mountaintop station.

The eight soldiers were in a group of 30 who were undergoing ski training and were close to the mountain’s peak when they were hit by the volcanic rocks, defence officials said.

The officials had originally said the injuries were caused by an avalanche, but later corrected that account.

One of the soldiers, who was slightly injured, said he took refuge in a forest by the ski slope after seeing black smoke and volcanic rocks shooting into the air. When visibility improved about 10 minutes later, he found several fellow soldiers fallen to the ground, Kyodo News reported.

A helicopter takes part in the rescue operation (Muneyuki Tomari/AP)

The four civilian skiers did not have life-threatening injuries, Gunma prefectural disaster officials said.

Only the crater had been off-limits because of low-level volcanic activity before Mount Kusatsu-Shirane erupted around 10am. The Japan Meteorological Agency has since banned access to the mountain.

The agency said the eruption and avalanche could not be linked immediately. Snow conditions and seismic activity are some of the possible causes of an avalanche. The eruption occurred in an area considered less at threat from volcanic activity, and officials were caught off guard, agency official Makoto Saito told reporters. No warning had been issued to visitors.

Defence Minister Itsunori Onodera said five of the eight soldiers were seriously injured. One of them, a 49-year-old male soldier, later died, said Wataru Tatsukawa, an official at the regional military training camp.

Mr Onodera said the training was for rescue operations in the snow, and it was regrettable that those who were supposed to help had to be rescued.

Town officials said everyone on the mountain had been accounted for, and all of about 80 skiers who took refuge at a gondola station at the top of the ski slope were brought down to the foot of the mountain, some by a military helicopter, others by snowmobiles. The rescue operation was finished by the evening.

NHK public television showed the first group of rescued skiers, wearing helmets, being handed a bottle of tea each and escorted into a cabin.

“I was scared to death, and I’m so relieved to come back alive,” an unidentified male skier told NHK, still wearing a helmet. He said he was inside a gondola with two of his friends when the volcano erupted. “Volcanic rocks rained down on the roof of the gondola. It shook so violently that I was afraid the whole thing might smash down on the ground.”

Other skiers said rocks as big as lunch boxes rained down.

Japan sits on the Pacific Ring of Fire and is prone to earthquakes and volcanic eruptions.